1. Technical Field
The present invention generally relates to wireless communication systems, and particularly relates to message detection, such as HS-SCCH Part 1 detection in a Wideband Code-Division Multiple Access system.
2. Background
High-speed downlink packet access (HSDPA) was introduced in Release 5 of the 3rd-Generation Partnership Project (3GPP) standards for 3rd-generation mobile phone systems to provide enhanced data transfer speeds to Wideband Code-Division Multiple Access (W-CDMA) mobile terminals. HSDPA is supported by a transport channel, called the High-Speed Downlink Shared Channel (HS-DSCH), a physical control channel, called the high-speed shared control channel (HS-SCCH), and a physical data channel, the high-speed physical downlink shared channel (HS-PDSCH). The HS-SCCH carries all of the signaling related to the HS-DSCH, while HS-DSCH user data is carried on the HS-PDSCH.
Recent revisions to the HSDPA specification, providing further enhancements in system capacity and mobile terminal efficiency, include the addition of support for advanced multi-antenna technology, i.e., Multiple-Input-Multiple-Output (MIMO) technology, as well as for so-called Continuous Packet Connectivity (CPC). These revisions each comprise extensions to the content and usage of signaling over the HS-SCCH.
CPC is introduced in W-CDMA Release 7. The original objective of the specification developers was to reduce the uplink noise rise caused by transmission of physical control channels for packet data users, to make it possible to keep more packet data users in a simultaneously connected state (the CELL_DCH state) for long time periods of time without reducing the cell throughput. Packet data users will thus experience significantly reduced delays, as time consuming reconnections are avoided, resulting in a user experience more similar to that currently experienced in fixed broadband data networks. However, the objectives of CPC have become broader, and now include a reduction of the downlink overhead as well as reduction of power consumption by packet data users' mobile terminals.
The new features of CPC are supported by a new cyclic redundancy check (CRC) computation method (Type 2 CRC) for the HS-DSCH and a new HS-SCCH message format (Type 2 HS-SCCH). Importantly, mobile terminals supporting Release 7 should also be capable of simultaneously handling the legacy (Release 6) HSDPA formats, referred to as Type 1.
One of the new features of the CPC revisions to the HSDPA specification is a so-called HS-SCCH-less operation. HS-SCCH-less operation is intended for low data rate applications such as voice-over-Internet-Protocol (VoIP) service. The general approach is to reduce transmission of the HS-SCCH, which normally generates a significant amount of overhead for small data packets, by eliminating the transmission of the HS-SCCH during initial transmissions of data. (Upon the failure of the mobile terminal to acknowledge successful receipt of an initial transmission of data, up to two Hybrid Automatic Repeat Request (HARQ) retransmissions may follow. Each of these retransmissions is “announced” using Type 2 HS-SCCH signaling). Because initial transmissions of new data are not signaled by the HS-SCCH in HS-SCCH-less operation, a mobile terminal must employ “blind” detection, using a limited set of coding parameters and transport block sizes, to detect these transmissions.
HS-SCCH-less operation is configured by the serving Radio Network Controller, or SRNC, on a per-user-equipment basis, by assigning four predetermined transport block sizes and two predetermined HS-PDSCH codes to a given user equipment (UE). The configured UE thus attempts to blindly decode all packets received on one or two HS-PDSCHs, subject to the limited choice of transport block size, using the new Type 2 CRC to detect successful decoding of an HS-SCCH-less transmission. At the same time, however, the UE should continue to monitor up to four HS-SCCHs for control messages directed to the UE. A Release 7 UE should detect both Type 1 and Type 2 HS-SCCH messages, and should also be able to determine whether a received HS-SCCH is of Type 1 or Type 2.
An HS-SCCH subframe (of any type) consists of two parts, transmitted over three slots. Part 1, which is mapped to the first of the three slots, is used by a UE to determine information describing an imminent HS-PDSCH transmission for the UE, including an identification of the channelization code(s) and modulation method to be used on the HS-PDSCH transmission. Part 1 messages targeted to a specific UE are distinguished from others through the use of an UE-specific scrambling sequence. Part 2, which is mapped to the second and third slots of the HS-SCCH subframe, provides information about transport block size as well as HARQ process information. Part 1 of the HS-SCCH subframe, which is transmitted slightly before the corresponding HS-PDSCH subframe, is designed to include enough information to enable the UE to begin reception of the corresponding HS-PDSCH subframe, as the transmission of Part 2 overlaps transmission of the HS-PDSCH subframe. The HS-SCCH is also used to give orders in exceptional cases—these orders are identified by a Part 1 message consisting of only zeros. For non-MIMO operation, Part 1 consists of 8 data bits coded to 40 bits (⅓ convolution code, rate matched to 40 bits) masked with a 16-bit UE identity coded to 40 bits (½ convolution code, rate matched to 40 bits).
MIMO is also introduced in Release 7. Like HS-SCCH-less operation, MIMO mode is configured per UE. Another new HS-SCCH format (Type M) is introduced to support MIMO. MIMO operation is a distinct mode; thus, a UE configured to be in MIMO mode should only detect and support HS-SCCH Type M and not Type 1 and Type 2. An HS-SCCH Type M subframe is split into two parts as before, but the number of bits and their meanings are different. In particular, Part 1 consists of 12 data bits instead of 8, although the 12 bits are coded to 40 bits for transmission, like Type 1 Part 1 messages.
A conventional HS-SCCH decoder might apply convolutional decoding, e.g., using a Viterbi decoder, to each part of the message. The decoded Part 1 message is used to configure receiver circuitry for demodulation and soft value extraction of the HS-PDSCH. Upon receiving Part 2, the CRC may be used to determine that the current message was for the given UE. Since up to four HS-SCCH codes (channels) are simultaneously monitored, a Part 1 message is decoded for each. Of course, only one of the messages may be targeted at the UE for any given subframe. Typically, the message corresponding to the “best” decoding result is used to set up for traffic data reception. A drawback to this method is that traffic data is always buffered and at least partially demodulated, regardless of whether there actually was a message for the given UE or not.
To save decoding resources and to allow HS-PDSCH reception to be shut down when there is no data for the given UE, various improved HS-SCCH detection methods have been proposed. In one approach, a received Part 1 coded bit sequence is not explicitly decoded, but is instead detected using a maximum-likelihood (ML) process. Given a Type 1 HS-SCCH, a 40-bit hypothesis sequence is computed for each of the 256 possible 8-bit Part 1 messages. Each of the 256 hypotheses is correlated with the received soft values on each of the up to four different HS-SCCH monitored. The hypothesis yielding the highest correlation value is selected, and reception of the corresponding HS-DSCH is begun if this correlation value is sufficiently large. In a variant of this approach, reception of the corresponding HS-DSCH is only initiated if the highest correlation value is sufficiently greater than the other (255) correlation results. For instance, the UE might require that the “winning” correlation result be at least τ times the average absolute correlation among the other candidates. The choice of τ is a trade-off between missed detections, which lead to reduced throughput due to re-transmissions, and false alarms, which increase UE power consumption due to unnecessary demodulation of the HS-PDSCH. The value of τ must of course be greater than one; a reasonable selection for the value of τ in some implementations might be 4.5.
With respect to Type 1 HS-SCCH signaling, the maximum-likelihood detection process described above is computationally more efficient than the conventional Viterbi decoding approach. It also yields a useful reliability measure for the preferred hypothesis, which may be used to decide whether Part 2 detection and traffic data demodulation is necessary. This in turn leads to reduced power consumption by the UE. However, the ML detection solution for Release 6 implementations (Type 1 HS-SCCH) needs to consider only 256 hypotheses (40 bits each) corresponding to the 8 bit data field in Part 1. Release 7 implementations that support HS-SCCH Type M would instead require 4096 hypotheses (40 bits each) corresponding to the 12 bit data field in Part 1. This makes ML detection of the HS-SCCH Part 1 message less attractive as an alternative to the convolutional decoding approach.
The maximum-likelihood detection solution for HS-SCCH Part 1 messages described above considers a trade-off between missed detections and false alarms. With the addition of Type 2 HS-SCCH signaling, false alarms will result in not only increased power consumption, as the UE attempts to decode non-existent Type 1 HS-DSCH, but will also reduce throughput in the event that a HS-SCCH-less transmission was actually scheduled for the HS-DSCH subframe corresponding to a HS-SCCH false alarm. To save power and avoid false alarms in detecting Type 1 Part 1 messages, a decoding approach is needed that facilitates the assessment of the likelihood that a decoded message actually corresponds to a message sent to the given UE, and allows false alarm and missed detection probabilities to be tuned to acceptable levels.